This invention relates to aqueous dispersions of polymers and to methods for the preparation thereof.
Aqueous dispersions of polymers have been prepared in the prior art by emulsion polymerization methods as well as suspension polymerization methods. The former methods are limited to the use of monomers which polymerize under emulsion polymerization conditions and further to applications which can utilize dispersions having average polymer particle diameters less than about 0.5 micron. The latter methods are sometimes undesirable because the resulting dispersions have undesirably large average particle diameters.
Many known and potential applications require aqueous dispersions which, for one reason or another, are not easily prepared by either emulsion or suspension polymerization methods. Various methods have been developed to prepare such dispersions. For example (as shown in U.S. Pat. Nos. 2,653,919; 3,055,853; 3,356,629; 3,432,483; 3,642,676; 3,746,681; 3,847,886 and 3,879,324) the polymer has been dispersed directly into the aqueous phase, usually with the aid of a solvent for the polymer and an emulsifying agent. Such methods are disadvantageous because the solvent is expensive, hazardous and difficult to recover for reuse. Further, residual solvent often left in the polymer impairs polymer properties. Moreover, if large quantities of solvent are not used in such direct dispersion methods, a very difficult mixing problem is presented which requires high temperatures and intensive agitation (with correspondingly high energy usage) to achieve adequate dispersion. High temperatures for prolonged periods often degrade the dispersing agents as well as the polymer. More importantly, the particle sizes of the dispersed polymer are not uniform and are difficult to control.
Indirect dispersion processes (often called inversion processes) have also been used to prepare polymer dispersions. In a typical inversion process, a water-in-polymer emulsion is first prepared by adding, with agitation, a small amount of water to a liquefied polymer medium. Subsequently, the water-in-polymer emulsion is inverted to a polymer-in-water emulsion by adding water or an aqueous solution of an emulsifying agent. See, for example, Canadian Patent No. 876,153 and U.S. Pat. Nos. 2,872,427 and 3,867,321. These processes also present difficult mixing problems and accordingly also suffer from such deficiencies as poor particle size control, long agitation periods, high temperatures, use of expensive solvents and large energy input. In addition, such processes are generally practiced in a batchwise manner usually involving considerable loss of polymer due to equipment cleanup at the end of each run or due to unsuccessful runs.
In view of the aforementioned deficiencies of the prior art methods, it would be highly desirable to provide an improved process which can be easily operated in a continuous and efficient (as to energy usage and as to raw material losses) manner to provide aqueous dispersions of polymers having controlled, uniform particle size.